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Composites Science and Technology 160 (2018) 169e198

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Composites Science and Technology

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Shape memory polymers for composites

Tong Mu a, 1, Liwu Liu a, 1, Xin Lan b, Yanju Liu a, *, Jinsong Leng b, **

a Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), P.O. Box 301, No. 92 West Dazhi Street, Harbin 150001, People'sRepublic of Chinab Centre for Composite Materials, Science Park of Harbin Institute of Technology (HIT), P.O. Box 3011, No. 2 YiKuang Street, Harbin 150080, People's Republicof China

a r t i c l e i n f o

Article history:Received 7 May 2017Received in revised form7 February 2018Accepted 14 March 2018Available online 22 March 2018

Keywords:Shape memory polymerShape memory polymer compositeConstitutive theoryStimulationAerospace4D printingOrigami

* Corresponding author.** Corresponding author.

E-mail addresses: yj_liu@hit.edu.cn (Y. Liu), lengjs1 These authors contributed equally to this work.

https://doi.org/10.1016/j.compscitech.2018.03.0180266-3538/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Shape memory polymers (SMPs) are a class of active, deformable materials that can switch between atemporary shape, which can be freely designed, and their original shape. With their large deformation,low density, various stimulation methods, good biocompatibility and other advantages, SMPs havebecome widely accepted as smart materials. However, SMPs have many limitations and weaknesses thatare exposed in engineering applications. For this reason, the significance of SMP composites (SMPCs) hasbeen analyzed in terms of four aspects: reinforcement, innovation and improvement of driving methods,the creation of specific deformations and the creation of multifunctional materials. We then introducethe constitutive theory of SMPs and the post-buckling analysis of SMPCs. Afterward, we introduce theextensive applications of SMPCs in the fields of aerospace, biomedical equipment, self-finishing,deformable mandrels and the 4D printing of active origami structures, demonstrating their ability toundergo active driving and deformation, their adaptiveness, their ease of transport and their rapidproduction capacity, which fully demonstrate the unique advantages of SMPs in solving applicationproblems. Finally, the advantages and disadvantages of SMPCs in applications are summarized, and theprospects for new SMPCs and new SMPC structures are described.

© 2018 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1702. Basis of shape memory effect in polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

2.1. Process of shape memory effect in polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712.2. Principle of shape memory effect in polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1722.3. Shape memory behavior under global view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

3. Why use SMPCs instead of SMPs? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1733.1. Reinforced SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1733.2. Novel stimulation based SMPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

3.2.1. Electric stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743.2.2. Magnetic and light stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743.2.3. Wetting stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

3.3. Creating SMPCs with novel shape memory behaviors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1763.4. Multifunctional material systems based on SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

3.4.1. Self-healing SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1783.4.2. SMPC surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

4. The mechanics of SMPs and SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1804.1. Constitutive theory for shape memory polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

@hit.edu.cn (J. Leng).

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198170

4.2. Post-buckling analysis of SMPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825. Applications of SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

5.1. Applications in aerospace and aviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1845.2. Biomedical devices based on SMPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1865.3. Two types of enlightening applications: smart textiles, microelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1865.4. A potential share: 4D printing and origami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

6. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

1. Introduction

The shape memory effect (SME) is a special mechanical phe-nomenon usually described by the shape memory cycle (SMC).Fig. 1.1 shows a highly common shape memory cycle. SMC

Fig. 1.1. Once flowers have opened for the day, can the material revert to its originalform again? This return can be achieved by shape memory effects. (a) This shapememory cycle consists of two hot stages (red background) and two cold stages (bluebackground), and the shape changes occur during the hot phase. First, the originalshape (straight) is heated and can be adjusted by an external force into a special shape(pentagonal). This shape can be fixed after cooling. This stage is called the program-ming process, and the temporary shape is called the programming shape. Whenheated again, the material will return to its original shape and output a certain re-covery force. This stage is called the recovery process. (b) Shape memory effect is easyto understand by an analogy. A spring and a telescopic rod with a lock are connected inparallel. When locked, the spring can be stretched without external force. Conversely,when unlocking, the spring restores the original length. We are about to find that thespring (entropy-based elastic network) and the shape lock have corresponding fea-tures in the polymer. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

represents the mechanical process that embodies the shapememory effect of that kind of material, and a more exact definitionwill be explained later. The shape memory effect is common inpolymers, but most examples are inferior. One class of polymerscan provide excellent shape memory effects and they are pre-dominantly used in shape memory applications. They are calledshape memory polymers (SMPs). Shape memory polymers areactive deformable materials that undergo large deformations, firstmentioned by Vernon et al. in a dental patent in 1941 [1]. In thesixties, heat-shrinkable tubes entered the market [2,3]. Their ap-plications have drawn substantial research attraction. As the se-venties began, a number of commercial companies developed theirown shape memory polymers [4]. Since the end of the last century,researchers have begun to systematically study shape memorypolymers. The principle of shape memory has been increasinglyelucidated, and diverse shape memory effects have been observed[5].

Taking the initiative to change shape is vitally necessary foranimals and even other organisms. Compared to inorganic ceramicsand metal materials, polymer materials show natural advantagessuch as lower density, better biological and organic compatibility,and easier modification and processing. Accordingly, shape mem-ory polymers are blossoming in radiant splendor in the field ofactive polymers. Actively moving materials can be effectivelydeformed in shape by external stimulation [6e9]. Examples such asshape memory polymers, electroactive polymers [10,11], photo-induced polymers [12], and hydrogels [13,14] have been the sub-ject of substantial research. Actively moving materials are oftencategorized by response behavior, and the differences in propertiesbetween different actively moving materials are significant.

Shape memory polymers are materials driven by externalstimuli that actively switch between multiple shapes. Comparedwith other materials, shape memory polymers have the advantagesof high stress tolerance [15], the ability to undergo large de-formations [2], a rich selection of driving methods (including heat3456, light [7,8], electricity [9e11], magnetism [12,13] wetting [14],and pH [16]), excellent radiation resistance and good biocompati-bility [17], which make them a research hotspot in the field ofactively moving materials.

At present, shape memory polymers have many applications inaerospace [18], medicine [19e21], self-finishing smart textiles[22,23] and electronic devices [24], and self-assembling structure[25e27]. Specific applications, including low-impact releasemechanisms in the aerospace field, large spatial deployable struc-tures, shape memory polymer sutures, minimally invasive surgicalinstruments with good biocompatibility, the active deformation orself-finishing of textiles, electronic devices, and variable mandrels,which effectively solve the thorny problems of the correspondingfields, show the great power of shape memory polymers. The aboveapplications involving composites will be described in detail inSection 5.

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 171

2. Basis of shape memory effect in polymers

In this section, the basic characteristics of shape memory poly-mers are reviewed, and the topic of shape memory polymer com-posites is not addressed. Themajority of this section is thus a classictopic, which can be found in a standard shape memory polymerreview. Therefore, this section is brief. More detailed descriptioncan refer to the supplementary documents.

This section does not discuss an important property of the shapememory polymer, the stimulation method. Because taking intoaccount shape memory polymers that used in engineering arecommonly based on phase change. Unless the compositematerial ismade, the driving method is scarce. So please refer to the relevantreview of shape memory polymers [28], and Section 3.2.

Shape memory polymers can be divided into physical bondingand molecular/supramolecular switching through the mechanismof switching, but also through the shape memory effect classifica-tion: Shape memory polymers can be divided into one-way andtwo-way from directionality of the shape memory effect (So far,two-way SME is reported only in shape memory alloys [29e31]), ordivided into dual SME and multi SME from the number of stableshapes. There are many delicate distinctions involved. Since ourfocus is on dual SMP based physical bonding which is convenientand mature for engineering applications, we will not cover them indetail in the text, please refer to the supplementary document.

2.1. Process of shape memory effect in polymers

Therefore, the shape memory effect involves comprehensiveeffects of material and process. We then describe the principle ofphase transition shape memory polymers, which allows us to quiteaccurately describe the shape memory cycle of polymers with astandard linear solid (SLS) model [32e34], as shown in Fig. 2.1 (a).The points on the corresponding stress-strain-temperature curvesfor each state are shown in Fig. 2.1 (b). We describe a hot

Fig. 2.1. A typical hot programming, hot recovery, unloaded dual-shape memory cycle andindicates that the viscosity of the dashpot is at its maximum (at room temperature) and thatthe dashpot is barely measurable (at high temperature) and that the material is highly elasshape distorted under external load into a temporary shape; D, application of an external loaand the occurrence of a small ordinary elastic recovery, not substantially altering the tempretation of the references to colour in this figure legend, the reader is referred to the web

programming, hot recovery, no-load dual-SME. We identify themost obvious mechanical characteristics of the shape memorypolymers. At high temperature, the polymer shows viscoelasticbehavior, while it shows general elastic behavior at low tempera-ture, which shows that viscosity has a great dependence on tem-perature, i.e., the viscosity changes between the maximum and asmaller measurable value during the phase transition process. Thisis the essence of the shape memory effect. First, upon heating, theviscosity of the dashpot is reduced to be barely measurable, andcreep occurs under an external load to obtain a programmed shape.When the shape is maintained by the external force and the tem-perature is decreased below the transition temperature range, theviscosity of the dashpot increases greatly. Then, when the externalforce is removed, apart from a small general elastic response, theprogrammed shape stays the same, and the shape creation processis now complete. When heated again above the transition tem-perature range, the viscosity of the dashpot decreases, and thesystem creeps back to its original properties. This descriptionsummarizes the main mechanical processes of the shape memoryeffect, but it is difficult to provide an accurate description of thedetails of the process, which makes improving the linear visco-elastic model challenging. A new description (the phase transitionmethod) has been proposed, and amore precise constitutive theoryof shape memory polymers will be presented in Section 4.1.

The recovery process of the shape memory polymer can exert aforce to the outside, so the shape memory polymer has a drivingability. But in this process, the energy is stored through mechanicaldeformation in programing process, while the external stimulus isonly used to open the switch. So we do not use the word “drivemethod”, which is prone to ambiguity, when talk about the stim-ulus later.

The above shape memory cycles (i.e., hot programming, hotrecovery, dual-SME without load) are used to evaluate whether theshape memory effect is good or bad, which is usually achieved bydescribing some representative parameters of the shape memory

the SLS model corresponding to the state of each stage. In this figure, the blue dashpotthe material has ordinary elasticity, while the red dashpot indicates that the viscosity oftic. Symbol legend: A, the original shape; B, the heated original shape; C, the originald to maintain the temporary shape as the material cools; E, removal of the external loadporary shape. A-B-C-D-E, programming process; E-B-A, recovery process. (For inter-version of this article.)

Fig. 2.2. Some cases of global deformation of shape memory polymers. In this figure, different colors represent different tensile states. The order of recovery can be designed to takeadvantage of different transition temperatures (or other stimuli) and the material can be kept in a certain state of local recovery, thus enriching the shapes of the shape memorypolymer that can be fixed. For the sheet, the creation of complex shape memory behavior has two basic ideas, namely, local recovery in the direction of thickness and local recoveryin the direction of length or breadth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198172

cycle, namely, the fixation rate and recovery rate. Through theshape memory cycle diagram, they can be expressed as follows(considering only the one-dimensional case):

Rf ¼εtem;t � εori

εtem;0 � εori� 100%

Rr ¼ εtem;t � εrec

εtem;t � εori� 100%

where εtem;0 is the strain at the last moment before the externalforce is removed, εtem;t is the strain at themoment of measurement,εori is the strain before the programming, and εrec is the strain afterthe recovery. This is also true for each SMC of multi-SME.

It should be noted that the reduction in the fixation rate consistsof two parts, the general elastic shape recovery due to the removalof external loads at the end of the program and the cold recoveryafter long-term storage. Cold recovery is related to the material andits storage conditions, such as temperature, external force, andchemical environment. The higher the temperature and the greaterthe external forces that tend toward the original shape, the morerapidly cold recovery will occur [28,35,36]. The chemical environ-ment may also trigger a recovery, such as by wetting [36]. The re-covery rate is dependent on the temperature. Additional, thefixation rate and recovery rate reflect the performance of the ma-terial in a specific shapememory cycle. Obviously, they are process-dependent values, which are related to strain, temperature, timeand even strain rate [37].

2.2. Principle of shape memory effect in polymers

For further explanation from the perspective of molecular me-chanics, the shape memory effect in a shape memory polymer-based glass transition is mainly due to the two-phase structure ofthe polymer: the stationary phase to maintain the original shape ofthe macrostructure and the reversible phase of softening andhardening. When the temperature is lower than the Tg range, theshape of the reversible phase can be frozen; otherwise the entropicelasticity drives the polymer network to recover to the initial state.

The stationary phase ensures that the network deformation isaffine without viscous flow due to external forces. In general,physical crosslinking and chemical crosslinking correspond tothermoplastic shape memory polymers and thermosetting shapememory polymers, respectively. When the Tg range of the sta-tionary phase is high, and softening and relaxation do not occur inthe temperature range of use of the material, so the memory of theoriginal shape can be ensured. When the Tg range of the reversiblephase is low, softening and hardening can easily occur as thetemperature changes, and the segment of material has a highdeformation capacity at higher temperatures. When the tempera-ture is low, the polymer network is in a low-energy state, thesegment cannot rotate freely, and the polymer is generally elastic atthe macroscopic scale. When the temperature gradually increasesto the Tg range, the segment rotation is unlocked, and then thepolymer is highly elastic on the macroscopic scale, with a highdeformation capacity. When the temperature is lowered again tobelow the Tg range, the segment does not return to the low-entropystate as the segment rotation locks, so the material shows a tem-porary shape on the macroscopic scale. The origin of the shapememory effect described above has been supported by recent ex-periments [38e42].

2.3. Shape memory behavior under global view

Finally, it is emphasized that the above discussion focuses on thelocal mechanical properties of shape memory polymers. The globalshape of the shape memory polymer is the result of the super-position of each local deformation. Therefore, because the tem-perature and stress distribution can be designed in time and space,the shape memory behavior of the global shape is quite complexand rich, as shown in Fig. 2.2. In the case of an average dual shapememory polymer, when thematerial is uniformly heated, the shapeof all the different parts of the material is restored together, that is,a one-step recovery process; when non-uniform heating occurs,the recovery of the different parts of the material is carried outseparately, i.e., “pseudo multi-SME" [43e45]. (When a deformationof the material is observed whose macroscopic behavior resemblesa kind of shape memory effect but does not accord with all the

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 173

stability requirements of the shape memory effect, for example, ifits shape is dependent on the amount of stimulation, we use theprefix “pseudo”.) The shape-memory behavior will be furthercomplicatedwhen external constraints occur; for example, “pseudotwo-way SME” may appear [46,47]. These complex shape memorybehaviors will create a series of smart shape memory polymerstructures, which will be described in Section 3.3.

3. Why use SMPCs instead of SMPs?

In general, the significance of compositematerials is that severalcomponents complement each other, result in synergies, orimprove or enrich the function of the matrix material. For shapememory polymers, composites are made with two basic objectives,i.e., reinforcement and finding new and effective stimulationmethods. (For shape memory polymer composites, the stimulationcan be more precise and highly selective, so more sophisticatedshape memory behaviors can be achieved, such as the example inSection 3.3.). In addition, multifunctional and smart shape memorypolymer materials are promising research directions, with abilitiesincluding self-healing [48e50], drag reduction [51], and opticalproperties [52e55].

This section describes shapememory polymer composites, whichcan be used as a matrix, filler, or layer of composites. When a shapememory polymer is used as amatrix, the doping can consist of one ormore of zero dimensions (particles), one dimension (fibers and ves-sels) and two dimensions (layers and films) of carbon materials,polymers, metal and ceramic particles, liquid or even gas, to achievereinforcement, novel stimulation methods, self-healing and otherfunctions. A shape memory polymer can also be used as a functionalfiller in the form of fibers or the like to form a multifunctional com-posite material. A well-designed composite structure can achievetwo-way recovery and other interesting and useful functions.

The study of shape memory polymer composites is driven byapplication, according to the need to design the type and propor-tion of filler reasonably. Too much filler will often reduce thethermal and mechanical properties and lead to a lower transitiontemperature. On the other hand, only a small number of fillers canenhance the shape memory effect; most fillers will reduce theshape memory effect of a polymer, as reflected in the fixation rateand recovery rate (see Fig. 3.1).

Fig. 3.1. Why make shape memory polymer composites? Reinforcing and providing more stimuli are two basic purposes. Recently, there has been a new purpose in makingcomposites.

3.1. Reinforced SMPCs

Shapememory polymers are relatively recent compared to shapememory alloys (SMA). Shape memory polymers were first proposedand noted in the biomedical field for their polymer-based properties[56]. In the field of engineering, shape memory alloys were exten-sively studied in US Navy laboratories in the 1960s [57]. Shape

memory polymers have greater deformation capacity, lower density,lower production costs, richer properties and modification methods,but because of limitations of their stiffness, strength and drivingforces, they are difficult to use as structuralmaterials. Given the needfor structures with greater deformability and lower weight, shapememory polymer composites are beginning to be studied.

We identify the differences between shape memory polymersand shape memory alloys. The shape memory effect of an SMAdepends on a martensitic transformation, i.e., martensite at lowtemperature and austenite at high temperature [58,59], whichleads to a higher Young's modulus at high temperature [57]. Theshape memory effect of a shape memory polymer is the diametricopposite, which allows the shape memory polymer to be pro-grammed much more easily than the shape memory alloy. Inaddition, the deformation of the lattice of a shape memory alloygives it clear superelasticity, as its restoring force is less than theYoung's modulus [57], while the shape memory polymer has highelasticity above the transition temperature, and its restoring forceand modulus are similar. In terms of their shape memory effect, ashape memory alloy has a unique two-way shape memory effect[60e64], while a shape memory polymer can have a multi-shapememory effect [65,66]. In terms of the stimulation method, shapememory alloys can be stimulated by heat or magnetism [67e70],while shape memory polymers have very diverse stimulationmethods, including heat, electricity, magnetism, light, ultrasound,solvents, metal ions, pH and others. Moreover, polymers can moreeasily be used to produce composites to achieve better propertiesand richer functionality, and this article will detail them.

A shape memory polymer is reinforced mainly to increase thedriving force of the recovery process and to provide better carryingcapacity, which correspond to a high elastic modulus and a generalelastic modulus, respectively. In general, the reinforcement filler im-proves these two indicators at the same time. Reinforced shapememory polymer composites mainly include particle [71e74],nanofiber [75,76], short fiber [77,78] and long fiber [79,80] rein-forcement. Carbonmaterials havebeenwidely used in shapememorypolymer composites because of their good reinforcement, rich phys-ical and chemical properties and good combination with polymer-based materials. Examples include carbon nanotubes [71,74], gra-phene [72,73], and carbon fiber [75e78]. Carbon materials can alsoform self-assembled structures, such as carbon nanotube arrays [74]and carbon nanopaper [81,82]. In addition to carbon materials, glass

fiber [77,83], silica [84,85], Kevlar fiber [77] and spandex fiber alsoprovide excellent reinforcement, and doping with metals such as Nihas also been shown to provide reinforcement [11]. Generally, thefiller has a better reinforcing effect when it exhibits an orderedstructure. Usually, in shape memory polymer composites, the rein-forcing ability of long-fiber filler is greatest, followed by that of shortfibers and particles [86e88] (see Table 3.1).

Table 3.1Effect of doping in shape memory polymers.

pure shape memory epoxypolymers

carbon black, CNT long carbon fiber

Transition temperature ~105 �C slight reduction (slight rise inpolystyrene)

constant

Modulus at normal temperature 2.8 GPa slight rise, <3.5 GPa significant rise, >8 GPaModulus at high temperature (recovery

force)~10MPa slight rise, ~10 GPa significant rise, >3 GPa

Limit strain >50% slight reduction significant reduction (mainly throughbuckling)

Fixed rate and response rate ~100% ~100% reduction, difficult to exceed 95%

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The tensile strain rate in the longitudinal direction of the longfiber is generally less than 2%, which restricts the tensile defor-mation of the shape memory polymer. However, when the materialis bent, part of the system shows compressive deformation, and thefiber may undergo great geometric nonlinear deformation, i.e., apost-buckling phenomenon, to achieve greater longitudinalcompression deformation [89e93]. Compared to the traditionalreinforcement method, the theoretical basis of long fiber-reinforced soft matter is different [94e96]. Based on buckling andpost-buckling, long-fiber shape memory polymer composites,which are commonly used to integrate deformation and structurein one material, have been applied in spatial deployable structuresand actuators, such as spatial deployable hinges [82], trusses [97]and antennas [98].

In general, too much filler or filler with too strong a reinforce-ment effect can also reduce the shape memory effect. In general,too much or too strong reinforcing materials tend to reduce theshape memory ability while enhancing the mechanical propertiesof the polymer. After adding the reinforcing filler, the reinforcingmaterial in the temporary shape is in the elastic deformation state.This is a cold recovery process that reduces the fixation rate. On theother hand, the reinforcing filler can be prone to plastic yield in alarge deformation, forming a permanent plastic deformation andthereby reducing the recovery rate. In addition, the influence ofdoping on the glass transition temperature is uncertain. Forexample, doping with spherical nanoparticles such as Ni powderwill reduce the glass transition temperature of epoxy-based shapememory polymer [11], while CNF/MWCNT doping in styryl shapememory polymer has the opposite effect [99]. Chen et al. incor-porated C60, carbon nanotubes and graphene into shape memorypolymers to test their shape memory properties. The resultsshowed that C60 and CNT significantly increased the recovery ratebut had little effect on the shape recovery stress, while the gra-phene reduced the material recovery rate but improved the re-covery stress [100]. It is necessary to refer to the applicationrequirements to balance the appropriate form and proportion offiller. Ni et al. demonstrated that 10e20wt% is the optimum fiberweight fraction with very low residual strain during cyclic loading.

However, some fillers can perform dual functions, bothproviding reinforcement and increasing the shape memory effect.Such fillers generally rely on supramolecular systems. For example,cellulose whiskers and graphene oxide are both reinforcing mate-rials and both interact with the polymer network through hydrogenbonds in a reversible supramolecular effect, fixing the programmedshape [101,102]. In addition, the addition of self-organized nano-particles, such as carbon black, to the polymer [103] may also in-crease the fixation rate and recovery rate of the material.

3.2. Novel stimulation based SMPC

In addition to improving thermal and mechanical properties,

shape memory polymer composites are often responsible forproviding new stimulation methods for the recovery process andprogramming process. In the wide range of applications of shapememory polymers, there is a demand for non-thermal triggers,such as in aerospace structures and actuators and biomedical de-vices. More accurate and non-contact stimulation methods areimportant steps in the application of shape memory polymers,including electrical, magnetic, and optical responses. One solutionis to fabricate molecular switch shape memory polymers, but it isdifficult to design such materials from scratch, their properties areunpredictable, and their uses are constrained by production costsand processes. The second solution is indirect stimulation using afiller, which is effective and easy to achieve by doping withconductive materials, magnetically conductive materials, orabsorptive materials. In addition, shape memory polymers can bestimulated by solvent, ultrasonic, etc.

We introduce a typical solution to novel stimulation. Thedevelopment process and specific examples can be found in thesupplementary document.

3.2.1. Electric stimulationElectric stimulation is the most convenient and precise of all

kinds of stimulation. The use of current-driven shape has produceda great increase in the applications of shape memory polymers.Because of their restricted electronic structure and low conduc-tivity, intrinsically conductive polymers have never been reported.Conversely, polymers mixed with conductors have many advan-tages, such as high efficiency, stability and ease of design. The heatis fed into the polymer through the current, enabling the endoge-nous initiate recovery process. Carbon materials such as graphene,nanotube and nanofiber as well as nanoscale granules of metal canbe used for the conductive doping of polymers.

There are two issues that need to be addressed when designingan electrical stimulation of shape memory polymer composites[9,28]. First, the doped conductor is formed into a conductive three-dimensional network structure. For example, nickle powder isformed to chain under magnetic field [10,11], or using carbon blackto connect short carbon fibers can increase the conductance by 100times [104]. Second, to make the doped conductor more uniformlyand firmly incorporated into the polymer matrix, surfactant [105]and chemical modification are generally used. A review of specificexamples can be found in the supplementary document.

3.2.2. Magnetic and light stimulationMagnetic driving and light driving are two of the most typical

and practical methods of non-contact stimulation and areextremely suitable for applications such as biological and medicalinstruments where stimulation by direct contact is difficult. Thesetwo means of driving have two further types of approach, directdriving and indirect driving (mainly heating). Light-reactive shapememory polymers generally involve reversible chemical bonds,

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 175

such as the cycloaddition reaction of cinnamic acid [7,106,107].However, the polymer itself does not have a magnetic heating ef-fect, and as a result, magnetic heat driving can only be achievedindirectly. Other methods of magnetic driving have never beenreported. Magnetic drives are similar to electric drives and alsoaddress the problem of doping and matrix connection.

Magnetic driving is a typical approach to stimulation withoutdirect contact. The principle is the indirect heating of shapememory polymers by doping with magnetic materials, such asferrite and soft magnetic materials. This heating method is highlysuitable for medical instruments such as implanted shape memorypolymer supporters. In 2006, Lendlein et al. added magneticnanoscale particles to poly(ε-caprolactone) with shape memoryeffects and thereby made a new type of shape memory polymerstimulated by an alternating magnetic field12. Further, this methodcan be improved in a heating mode which mixes magnetic induc-tion heating and direct heating [108]. Since the magnetic heating iscontrollable, the required for ambient heating is tunable. In thisway, the material exhibits its apparent switching temperature be-ing varied. They also added magnetic materials to shape memorypolymer systems with temperature memory effects and obtainedshape memory polymer nanoscale composites with magneticmemory effects [109]. Leng and Smoukov et al. reported the mag-netic driving of a composite thin membrane made of Nafion andFe3O4, with the controllable realization of as many as four pro-grammed shapes13. This magnetic driving was highly controllable.Even when the local temperature exceeded 80 �C, the surfacetemperature remained near the body temperature (38e40 �C),which offered great potential in medical applications.

To address the problem that doped materials such as the mag-netic medium ferrite oxide had obvious boundaries with thecomposite and were unevenly distributed, modifications of theferrite oxide surface and crosslinking it into the composite networkprovided an effective solution. Lendlein et al. covered the surface ofmagnetic particles with oligo(u-pentadecalactone) (OPDL), cova-lently incorporating nanoscale metal granules into the composite,thereby obtaining even doping distributions, and achieved precisecontrol of a two-segment shape effect and a reversible shapememory effect through magnetic fields [110]. There are other waysto combine. For more examples, see the supplementary document.

Light driving is splendid for enabling remote driving and con-trol. Reactions can occur under different wavelengths of lightdepending on the doping, and light from all wave bands has thepotential to serve as stimulation. Table 3.2 lists some examples. Fora brief introduction to the following examples, please see thesupplemental documentation.

Table 3.2Doping absorption and the corresponding wave band and wavelength.

Doping Absorption band

Free radical reaction network UVGold nanoparticles/nanorods VisibleCarbon material RadarFerrite RadarWater IRYb(TTA)3Phen and Nd(TTA)3Phen IR

Infrared light and microwaves have good penetration ability,which makes them suitable for the driving of shape memorypolymer biomedical instruments. Electromagnetic waves in thiswave band mainly produce heat effects, and therefore the essentialprerequisite for using infrared light andmicrowaves is to establish awave-absorbing material. These materials are also frequentlyresearched as wave-absorbing materials for radar. The wave-absorbing agents of wave-absorbing materials mainly act throughresistance, an electric medium or a magnetic medium. Resistanceforms include simulated circuit dampers, carbon materials, metalpowders, silicon carbide, and other conductive materials. Electricmedium forms include barium titanate, conductive polymer ma-terials and other materials with high dielectric constants. Magneticmedium forms include ferrite, carbonyl iron, soft alloymagnets andother materials with self-resonance and hysteresis loss. Other re-views present more detailed discussions of microwave-absorbingmaterials [100,101]. Shape recovery by wave-absorbing agentssuch as nanoscale carbon materials [102,103], silicon carbide [123],nanoscale ferrite materials [104], water [105] and rare earth organiccomplexes 106 have been reported. Magnetic medium materialssuch as ferrite have the best wave-absorbing ability, but thecomposition boundary remains obvious because they are metal,and they decrease the thermomechanical ability of the shapememory polymer to undergo large deformations. Taking thereverse consideration, carbon materials, such as carbon nanotubes,carbon black, and carbon fibers have a wide absorption range.Additionally, their combination with polymer bases enhances thedynamic properties of shape memory polymers to some extent,which gives them abundant applications as wave-absorbing fillers.

3.2.3. Wetting stimulationAdditionally, solvents can serve as a method of stimulating the

shape recovery process. The doped solutions reported to driveshape memory polymers include a shape memory polymer with aglass state transformation and a hydrogen bond-locked shapememory polymer. For a shape memory polymer with a glass statetransformation, the essence of solution driving is plasticization. Asmoisture increases, the polymer chain parts are softened gradually,and the temperature of the glass state transformation decreasesbecause of the swelling effect and the polarization effect. For mostplastics such as epoxy resin, phenylethylene and other engineeringplastics, the speed at which solvents permeate them is very limited.In 2014, Leng et al. reported an epoxy resin shape memory polymerdoped with SDS that could undergo moisture driving [124].

On the other hand, the shapememory effect can be created fromscratch using composites. Qi et al. made a nanoscale polymer of

Absorption wavelength reference

~302 nm [111]~532 nm [112e116]Various (Broad absorption) [117e119]Various (Narrow absorption) [120]~1.9 mm [121]980 and 808 nm [122]

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198176

Poval and oxidized graphite, which adopted temporary shapesupon heating and underwent controlled shape recovery in mois-ture [87]. The switch phase of this shape memory polymer wasbased on hydrogen bonds, and both heating and solvation coulddestroy hydrogen bonds, unfreezing the macromolecular chainparts fixed at the surface of the oxidized graphene and allowingshape recovery. Zhu et al. added nanoscale cellulose whiskers toelastic thermoplastic polyurethane, and the material systemshowed shape memory effects with similar principles to the pre-vious example [88]. Zhang et al. designed a novel shape memorypolymer composite that adheres to a layer of cellulose nanocrystal-supported carbon black (CB@CNC) in the PU micrometer skeleton[125], which is equivalent to introducing a shape lock. This can beachieved by heating, moisture and various other means of stimu-lation. In the three instances above, Poval and polyurethanethemselves are entropic elastic networks, but they did not showshape memory effects. After the addition of nanoscale structuresthat could form hydrogen bonds, the systemwas frozen, and it hadshape memory effects at that time, which illustrates the origin ofshape memory effects to some extent.

3.3. Creating SMPCs with novel shape memory behaviors

As described at the beginning of this paper, the shape memoryeffect is a process-dependent phenomenon that depends onvarious thermodynamic processes, especially the spatiotemporaldistribution of temperature and external force. In the absence ofload, this diverse deformation process can be attributed to thezonal recovery of the material. The recovery of its global shape goesthrough several specific temporary shapes, and sometimes thesetemporary shapes can even remain stable. Phenomenologically,this example seems to be summarized as a multi-SME, but it mustbe noted that according to the definition of a multi-SME, theremustbe some temporary shape in the recovery process that remainsstable at a certain temperature. In this well-designed recoveryprocess, the shape is sensitive to the temporal and spatial distri-bution of heating, so we call it pseudo multi-SME. The most com-monmethod is to achieve non-uniform heating during the recoveryprogress, which will be described for two typical representatives.First, for pre-stretched (uniaxial or biaxial) shape memory polymersheets, heating one surface will cause the material to appear war-ped, resulting in a warping of the sheet. Second, for pre-programmed (including stretching and bending) shape memorypolymer strips, different portions of the strip are heated to achievecontrolled and segmented shape recovery.

For the creation of a specific temperature spatiotemporal dis-tribution, the simplest approach is partitioned heating [126e128].However, partitioned heating is difficult to control, so it is not easyto use. Therefore, it is important to introduce an accurate heatingmechanism, and the previously described shape memory polymercomposite, which is indirectly heated, again demonstrates its util-ity. When these dopants exhibit a particular distribution, such asdistribution at a surface or only in a certain area, they are heated ina fixed way. Dickey et al. stained a biaxial pre-stretched transparentshape memory polymer (Shrinky-Dinks) with dye at a specificlocation. When the polymer was heated by light, the black portionwas heated first, the painted portion unilaterally contracted, andthe sheet showed curvature45. This kind of bending is designable,so this method can create the required three-dimensional origamistructure.When heated further, thematerial will be fully recovered,showing a smaller planar shape. This group then analyzed the ef-fects of the power, intensity, width and shape of the laser used onthe folding [129]. Further, the group used different colors to dyedifferent parts and then used different colors of light for stimula-tion, thus achieving a selective response that facilitates the creation

of more complex shapes [130] (Fig. 3.2(a)). Xie et al. reported asegmented shape memory polymer composite strip constructedsequentially by ferrite doping and undoped and carbon nanotubedoping. Ferrite is heated by low-frequency electromagnetic waves,while carbon nanotubes are heated by high-frequency electro-magnetic waves. The undoped portion is finally recovered byheating, resulting in four different shapes [43] (Fig. 3.2(b)). Inaddition, through a similar construction process, segmentation bylight and heat control is also reported [131] (Fig. 3.2(c)). Guo et al.deposited carbon nanotubes on the shape memory polymer filmand described the local recovery and control response rates,depending on the location and number of layers deposited [132].

On the other hand, shape memory behavior will be furthercomplicated if we consider external constraints. We review somevaluable applications that have been reported. It is worth notingthat unlike the no-load case, an ingeniously designed externalconstraint can achieve stable temporary shapes. Light-inducedshrinkage is an important means of introducing these constraints,that will enrich the number of temporary shapes for opaquepolymers with photocrosslinking or photo-isomerous [133,134].Constraints can of course also be introduced by additional con-struction features, such as a laminate. Felton et al. reported a seriesof shape memory polymer origami structures [26,135,136]. Bothsides of the pre-stretched shape memory polymer are constrainedby a film that is only bendable, and a notch is formed in the portionto be folded. This topic will be explained in detail in Section 5.3.

In addition, a specially designed laminate can be used to fabri-cate a two-way deformation actuator whose two-way deformationis facilitated by the great changes in the modulus of the shapememory polymer during the process of phase transition. Note thatthe two-way deformation is not programmable. We introduceseveral representative designs. In 2010, Hu et al. first proposed theidea of two-way bending laminates and laminated polystyrenewith shape memory polyurethane, which was equivalent to addinga temperature-dependent constraint to the surface of a uniaxiallypre-stretched shape memory polymer sheet [46,137] (Fig. 3.2(d)).Imai designed a new two-layer structure, including one or twolayers of shape memory polymer and composite aluminum toachieve electrical driving [138]. Zhang et al. also achieved two-waybending with an electroactive polymer and a shape memorypolymer. The curvature was also calculated and verified by finiteelement simulation [139] (Fig. 3.2(e)). Leng et al. used shapememory polymer attached to an electroactive polymer and ach-ieved a degree of freedom for programmable two-way deformationby the double control of heating and voltage47. However, thesemethods can only achieve two-way bending, limited by the designof the special drive. Achieving designs withmore complex two-waydrive structures remains our future direction.

Recognizing that shape memory behavior is process dependent,we can design richer deformation processes for shape memorypolymers. Considering the design of the heating conditions andexternal constraints can greatly broaden the imagination space ofsmart shape memory polymer structures. Because of the high po-tential for the design of smart structures and of temperature andconfinement distribution in space and time, although shapememory behavior design is rich, even endless, its design andapplication to date have only exposed the tip of the iceberg andawait further development.

3.4. Multifunctional material systems based on SMPCs

Reinforcement and the search for new effective stimulationmethods are two basic goals of shapememory polymer composites.In addition, the filler can also confer new functions on the shapememory polymer, resulting in novel multifunctional composites.

Fig. 3.2. List of several ways to create novel shape memory behaviors using dual-SME. (aec) Pseudo multi-SME. (d,e) Pseudo two-way SME. (a) Different forms dyed with light. (b,c)Different doping. (d) Laminate polystyrene with SMP. (e) laminate SMP with EAP [43,46,114,115,123].

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 177

Fig. 3.3. Self-healing of shape memory polymers: sources of damage, self-healingstimulation and principles and assistance by shape memory polymer properties.

Table 3.4Summary of different reversible interactions as well as their applicability to self-healing and shape memory/change.

Reversible Interaction Self-healing Shape memory/change

Covalent crosslinking Yes YesIsomerization No YesMetathesis Yes NoMetal coordination crosslinking Yes YesHydrogen bonding crosslinking Yes YesSalt crosslinking Yes YesHeata Yes Yes

a Heat includes diffusion and phase transition.

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Multifunctional materials and structures are highly compact due tothe integration of different materials or devices and have the ad-vantages of saving weight and space, simplifying structural designand broadening the application scope of the materials [140].

Inspired by biology, researchers began to study smart materialsand structures, that is, multifunctional material systems that inte-grate perception, processing and feedback. When external stimulisuch as electricity are applied as input, shape memory polymercomposites can process them into heat, as described above, andoutput the feedback form as shape recovery. Therefore, electric-stimulated shape memory polymer composites can be consideredsmart materials.

In the following review, we consider the shapememory effect asa single function and focus on shape memory polymer compositeswith additional functions. As a composite is still a polymermaterial,the material or phase transition process will have many propertiesand functions, such as insolubility, insulation, refractive index, andthermal expansion and change. These properties are inherent, butin general, inherent properties do not attract our interest. However,in addition to these phenomena, the properties of artificiallydesigned phase transitions are also significant, such as changes incolor [141], permeability [142] and transparency [143].

As with phase transition properties, most of the properties ofcomposites are ordinary. However, the design of multifunctionalcomposite systems often exploits such properties. Typical examplesinclude self-healing materials, soft electronics, material healthmonitoring, energy collection and management, and radar ab-sorption. Because of the excellent properties of the shape memorypolymer itself and its working environment, self-healing propertiesare the most noticeable, the most compelling and the mostextensively studied, and therefore, self-healing shape memorypolymers and composites are reviewed first.

3.4.1. Self-healing SMPCsSelf-healing is a classic biological concept. Every animal and

plant has excellent self-healing ability to resist various possibleinjuries from the living environment. Similarly, materials are facedwith the threats of fatigue damage and accidental damage, such asimpact, bending, stretching, electric shock or chemical corrosion.Damage to traditional materials is irreversible, which increases costand also introduces many hidden problems. Therefore, self-healingmaterials are of continuous research interest, and materials such asconcrete [144,145], polymers [146,147] and ceramics [148,149] areextensively studied. Polymer systems (especially soft polymers)have inherent self-healing advantages, i.e., there are long segmentsthat can be moved and modified, so for a self-healing polymer, twoconcepts are essential: first, the polymers achieve re-crosslinkingthrough their own physical and chemical properties [150], andsecond, damage repair can be achieved by doping with a healingagent [151]. Polymer nanocomposites are a common approach andeither provide healing agents through microcapsules or microtu-bules or introduce a framework to provide an external force thatcan bring the fracture surface back into contact.

A large number of reviews have summarized the principles andexamples of self-healing [152,153] so this article only describes theprinciples and several examples related to shape memorypolymers.

Shape memory polymers have a close relationship with self-healing polymers, which is manifested in the shared re-quirements of the shapememory effect and self-healing ability. Theintersection of the two behavioral principles is shown in Fig. 3.3. Ashape memory polymer undergoes a large deformation in its workprocess and often plays the role of a structural material in appli-cations, which makes damage to the shape memory polymer moreprobable. On the other hand, the shape memory polymer has the

inherent advantage of self-healing without an external force, i.e.,the shape can be recovered after injury, and the fractured surfacewill recombine, allowing the material to achieve adhesion of thewound without external force after a limited degree of damage.This ability solves the problem that the traditional self-healingprocess requires external force bonding and thus offers high po-tential for engineering applications. In summary, the shape mem-ory and self-healing effects are promoted by similar properties, andthe development of self-healing polymers with a shape memoryeffect is highly attractive.

For a given polymer, the healing of cracks involves two phases.First, the surfaces of the cracks must remain in contact with eachother [154] and form adhesions between the surfaces by physical orchemical processes, which can be provided by the material or byadded healing chemicals. Due to their intrinsic well-behaved shapememory effects, non-destructive damage to shape memory poly-mers (where the effect on undamaged parts can generally beregarded as a process of cold programming) can often be recoveredby the intrinsic shape recovery properties in the first phase. Wenow introduce the second phase.

Self-healing can be achieved by the polymer itself, which hasalready been addressed by many summaries [130e132] and will bediscussed only briefly here. Generally, self-healing is based on areversible physical effect, that is, expansion occurring near a crack,or on reversible bonds, including reversible covalent bonds[155,156], ionic bonds [157], metal coordinate bonds [158,159] andsupramolecular interactions [160,161].

Self-healing materials operating by this kind of principle oftenrequire triggering conditions, such as heating, to promote the quickexpansion of the contact surface or to trigger chemical reactions. Itis noteworthy that the reversible bonds available for self-healingeffects are similar to the ones in shape memory polymers basedon reversible bonds, which enables many kinds of shape memorypolymers based on reversible bonds to exhibit self-healing ability.Below, Table 3.4 summarizes whether various kinds of photo-chemical principles can be applied to self-healing or shapememoryeffects. As illustrated previously, the principles of shape memorypolymer composites stimulated by new methods are similar tothose of this kind of light-stimulated self-healing material system[97,162].

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 179

Self-healing materials that rely on physical expansion aregenerally soft thermoplastic materials or poorly crosslinked rubbermaterials whose chain parts exhibit good expansion ability. In realapplications, many thermosetting shape memory polymers withhigh crosslinking density form a complex solid network usingchemical crosslinking. The interaction between the surfaces afterthey are broken apart is relativelyweak, and the speed of expansionof the chain parts is low, which hinders self-healing. Therefore, theuse of adhesive agents that are not part of the material itself is apromising idea. Through the reasonable design of the expansion ofthe adhesive agent, artificial healing could be transformed into self-healing of the material system.

Next, we discuss the effect of shape memory polymers on self-healing properties, i.e., the spontaneous completion of thesplicing process. This can be achieved by a shape memory polymermatrix or by shape memory polymer fiber doping. A large numberof molecular switch shape memory polymers have been previouslyreported to be self-healing, such as Diels-Alder reaction-basedshape memory polymers [163] and thermoplastic shape memorypolymers. For general thermosetting shapememory polymers, self-repairing abilities can be obtained by doping with a thermoplasticpolymer such as PCL. Luo et al. reported the addition of a PCL-basedshapememory polymer fiber with electrospinning to epoxy resin toobtain a self-healing shape memory polymer composite [48].

Depending on the actual application conditions, shapememory-assisted self-healing (SMASH) and close-then-heal (CTH) [50] aretwo kinds automatic splicing in self-healing polymers. The differ-ence between these two methods is mainly whether the shapememory part is programmed beforehand. SMASH is simpler tocreate than CTH, but CTH has a wider range of applicability. Theshape memory polymer is effective (close to 100%) when noambient load is applied, and therefore, the SMASH method can beused. On the other hand, when there is an external load, the shapememory polymer has difficulty in completing recovery, so the CTHmethod is very effective [130]. The basic process of CTH is shown inFig. 3.4. When a shape memory polymer matrix is used, the shapeof the polymers will recover to the original shape when there is noexternal constraint. Therefore, the preprogrammed shape memorypolymer base requires an external constraint upon recovery. A

Fig. 3.4. Schematic of self-healing progress of shape memory polymer-ba

fabric ] or grid can provide external compression constraints toachieve splicing.

In addition, CTH can be extended to other matrices. Dopingother active materials can also be the driving force for fracturejoints, including shape memory alloys [164,165], artificial muscles[166], and shape memory polymers [167e171]. Shape memoryalloy doping was the first to be reported [148,149]. Shape memorypolymers have better compatibility with polymer-based compos-ites than SMAs but have less driving force and require morestretching. There are several reports on self-healing stimulated bydoping with shapememory polymer short fibers [151e155]. Li et al.have predicted the mechanics of CTH by adding pre-programmedshape memory polymer short fibers [172,173].

3.4.2. SMPC surfacesThe development of shape memory polymer surfaces is an

attractive direction. Surface microstructure has been a topic ofgreat concern in recent years. The concept is biomimetic, that is, thenature and phenomena of biological or natural nanomaterials, de-vices and processes are derived from complex surface interactionsand physical and chemical properties [174,175]. These microstruc-tures greatly expand the specific surface of the materials, and thestructure even reaches photon size, thus achieving special me-chanical and optical properties that are geometrically related, i.e.,geometric deformation will produce a property change. Shapememory polymers have the basic property of variable geometry,based onwhich a large number of surface microstructure materialswith variable properties will be generated. The variable propertiescaused by geometric deformation can be divided into two cate-gories, affine deformation and non-affine deformation. Please notethat the criteria are local. The former is mainly based on tension orcompression, which leads to tuning properties [176], while thelatter is based on the creation or erasure, of surface microstructure,which leads to properties that either exist or disappear[52,160,177]. Superficial hydrophobicity [178], fluid drag reduction[51], transparency [179] and one- or two-dimensional photonicstructures [161,162] can be achieved by creating surface micro-structures of shape memory polymers such as waves, bumps, treeprotrusions, and column arrays.

sed smart foam, showing an example of close-then-heal (CTH) [50].

Fig. 3.5. Controlled wettability based on reversible micro-cracking on a shape memory polymer surface. (A) Schematic of the process of hydrophobicity change. (B) Microstructurephotos. (a,c) As the film cracks, the polymer substrate is exposed, so the surface shows a hydrophilic nature. (b,d) Metal cracks in contact with the surface of the material showingthe hydrophobicity of the metal [164].

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198180

When anothermaterial is attached to the surface of the polymer,such as a metal film, main surface microstructure is made of thefilm, the shape memory polymer is mainly provided geometricdeformation to achieve the change of the film. Some novel prop-erties can be achieved by composites, which is difficult for purepolymers. Leng et al. aluminized a shape memory polymer surfacewith artificial cracks to achieve tunable hydrophobicity [180]. Theprocess is shown in Fig. 3.5(A). In the original shape, a hydrophobicaluminum film completely covers the polymer surface. When thepolymer substrate is stretched, the aluminum film is separated bythe presence of cracks and is insufficient to cover thewhole surface.Therefore, the hydrophilic polymer matrix is exposed, and thecontact angle is significantly increased. Fig. 3.5(B) shows someevidence of this phenomenon using microstructures.

Polymer surface microstructures have three basic methods ofcreation: top-down creation, bottom-up creation and the combi-nation of the two. In the past, the top-down approach has domi-nated, that is, microstructures have been obtained by macroscopictreatment of the polymer surface, including lithography,imprinting, and the like. With the aid of the shape memory effect,bottom-up creation is possible, mainly by buckling of the film or

Fig. 3.6. Surface structure of shape memory polymer created by buckling. (A) one-dimensio(c) microstructure photo. (B) One-/two-dimensional sinusoidal corrugated surfaces. (a) Sche[54,55].

lines on the polymer substrate.Buckling is a long-studied structural stability problem. In past

practice, buckling has been avoided because it severely degradesthe load carrying capacity of the structure. In recent years, this viewhas been changing. This strongly avoided phenomenon has uniqueenergy and geometric characteristics, and with the rise of smartstructures in sensors, energy collection, damping, drivers and high-deformation structures, the subject has gained wide attention. Theuse of shape memory polymers can effectively produce buckling ofa film or line to facilitate the creation of surface microstructures,such as one-dimensional sinusoidal corrugated surfaces [54] ormore complex two-dimensional structures [55] (Fig. 3.6). This is animportant case of buckling mechanics, which will be followed by adetailed introduction to its theory and creation method.

4. The mechanics of SMPs and SMPCs

As mentioned earlier, the shape memory effect is a polymermechanical behavior. With the principle of shape memory effectexplained, increasing numbers of people are aware of the univer-sality of this effect. Once it is recognized that the shape memory

nal sinusoidal corrugated surfaces. (a) Schematic of production progress, (b) photo andmatic of production progress. (b) Uniaxial and biaxial recovery create different shapes

Table 4.1Several multi-component models of shape memory polymers.

Reference Model diagram

Tobushi 1997 [173]

Chen 1999 [174]

Diani 2006 [178]

Nguyen 2008 [179]

Westbrook 2011 [177]

Yu 2012 [182]

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effect is to a considerable extent a process property, the descriptionof the mechanical behavior becomes very urgent. There are sig-nificant differences in the shape memory behavior of polymerswith respect to different programming and recovery processes,which are manifested in the arbitrariness of the temporary shape,the length of the recovery time and the different forms of damageto the material.

For the calculations required in the application field, it isnecessary to supplement the constitutive relation of the shapememory polymer. In addition, for specific structures, specialsimplification is required for use. In this section, we first discuss theconstitutive model of pure shape memory polymers. The me-chanical performance of shapememory polymers in a typical shapememory cycle is presented in the Introduction, so we will notrepeat the description here. A suitable constitutive theory of shapememory polymers should successfully describe the shape memorycycle but, of course, is not limited to this. The modeling of shapememory polymers includes thermodynamic testing and mathe-matical description. Although the testing process is important, inthe application of the model, the conclusion is even more impor-tant. Therefore, this paper introduces several constitutive modelsfor shape memory polymers, including the viscoelastic model andthe phase transition model.

Compared with the research on constitutive models of shapememory polymer, the modeling of shape memory polymer com-posites began much later, with little research on their mechanics.Except for a few special structures, such as beams [181e185] orsheets [186e188], shapememory polymer composites are poorly oreven incompletely modeled, especially for dynamic predictions,which are difficult. For the further application of extension, it isurgent to establish models for accurately describing geometricaland mechanical behavior and for the dynamic analysis of shapememory polymer composites. It is hoped that this field can receivecorresponding attention.

4.1. Constitutive theory for shape memory polymers

The modeling began much later than the discovery of shapememory effects. In the 1990s, Tobushi et al. first modeled the shapememory behavior of polyurethane [189] using a four-elementmodel that adds a residual deformed element into a standardlinear solid model, describing the one-dimensional mechanicalbehavior of the shape memory polymer with incomplete recovery.Further, a non-linear correction can be made to the model [32].Chen et al. proposed the use of dual Maxwell elements in parallel todescribe this incomplete response [190]. Now, the viscoelasticmodel has been extended to 3D [191e194]. In 2006, Diani et al. firstproposed a three-dimensional constitutive model [178], whichsuggests that the elasticity is mainly generated by entropy orcohesion and gives a mechanical explanation of the stress-strain-temperature variation, but the details of the process are not suffi-ciently accurate. In 2008, Nguyen and Qi et al. proposed a newtheory that describes the behavior of shape memory more accu-rately, taking into account the effects of incorporating structure andstress relaxation from the perspective of the molecular mechanism[195]. In 2011, Chen and Nguyen developed the previous model andproposed a more complex description of the molecular mechanism[196]. In 2013, this group used multiple discrete relaxation pro-cesses to describe stress relaxation to study the effect of deforma-tion temperature and physical aging on the shape memorybehavior of amorphous networks [197]. In another work, multiplerelaxation times were considered [177,198], that is, multipleMaxwell elements were included to account for the time and

temperature dependency of the shape memory effect. Some typicalcomponent models are listed in Table 4.1. The finite elementmodeling of shape memory polymer structures has also beenproposed [199].

Another modeling approach, which was first used in shapememory alloys, is based on the phase transition process [200].Compared with metal, which has a clear phase boundary, thephysical and chemical properties of polymers change gradually, sothe polymer phase transition is a relatively fuzzy concept but doesindeed occur in the shape memory cycle as a phenomenologicaldescription, which allows us to use this modeling approach. Ac-cording to the discussion of Section 2.3, we know that the processof shape memory polymers across the glass transition temperaturerange can be described by the proportional function of the frozenphase ðFðTÞÞ. As shown in Fig. 4.1.

In 2006, Liu et al. first introduced the phase transition methodfor the shape memory behavior of polymers [201]. It is maintainedthat the shape memory polymers are a two-phase mixture near thetransition temperature. The strain is composed of mechanical strainand thermal strain, and the mechanical strain is composed ofstorage strain and elastic strain. Using the composite method, theshape memory polymer is described at different temperatures, butthe model is only suitable for one-dimensional small-strain pro-cesses. In 2008, Chen et al. considered large strain and extended themodel to 3D [202,203]. In 2009, Wang et al. introduced parametersrelated to heating rate and hysteresis into the improved model todescribe the complex shape memory behavior [204].

On the other hand, three-phase models have also been devel-oped to avoid the introduction of stored strain and to provide a

Fig. 4.1. Proportional function of the frozen phase ðFðTÞÞ. By this way, the process of shape memory polymers across the glass transition temperature range can be described. Thelonger segments are softer, and generally easier to unlock from the frozen phase. The black parts are also segments, their status is not investigated, but can be considered similarly.

Table 4.2Frozen phase volume fraction functions proposed in the existing literature. 191.

Reference Frozen phase volume fraction function

Liu et al. (2006) [185] FðTÞ ¼ 1� 11þcf ðTh�TÞn

Qi et al. (2008) [189] FðTÞ ¼ 11þexpð�T�Tr

a ÞWang et al. (2009) [188]

FðTÞ ¼ a exp��

�TtT

�m

b�n�

Reese et al. (2010) [210] FðTÞ ¼ 11þexp 2w

T�Tr

Volk et al. (2011) [211] FðTÞ ¼ b�tan T�AB

b�aGilormini and Diani (2012) [212]

FðTÞ ¼�1�

�T�Tmin

Tmax�Tmin

�m�n

Guo et al. (2015) [213]FðTÞ ¼ R T

Ts1

Sffiffiffiffiffi2p

p exp�� T�Tg

2S2

�dT

Li et al. (2016) [191]

FðT ; _TÞ ¼ 1�

0BB@1�

�1� exp

�� DHaðTÞ

kBT

��Dtro

1CCA,

R∞rcðTÞ pðrÞdr

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198182

more accurate description of shape memory behavior. In 2008, Qiet al. proposed a three-phase model of the three-dimensional finitestrain, which divides the glass phase in the traditional two-phasemodel into the original glass phase and the frozen glass phase[205]. This model provides a better prediction of shape memorypolymer properties during the cooling process. In 2010, Kim et al.developed a three-phasemodel (a hard segment, an active segmentand a frozen soft segment) based on the microstructure variationprinciple [206]. The stress and strain relationships of each phase aremathematically described using a three-element viscoelasticityequation and two Mooney-Rivlin hyperelastic equations to moreaccurately describe the shape memory behavior of polyurethane.

Table 4.2 lists a number of expressions for frozen volume frac-tions that introduce parameters to fit the shape memory behavior,but these parameters do not have strong physical significance, thatis, they do not obviously correspond to the known properties of thematerial. Li et al. proposed a physical model in 2016 [207]. But this

area still needs to continue to study, more accurate and universalmodel to be developed. Finding the physical explanation of thevolume fraction distribution is an important challenge for thefuture development of the phase transition method. In addition,phase transitions are also related to other mechanical conditions,such as stress [208]and strain rates [209].

4.2. Post-buckling analysis of SMPC

Driven by applications, surface-adhered thin films or wires,layered composite materials and fiber-doped shape memorypolymer composites have drawn attention for use in substitutionstructures and in driving bi-functional materials. Shape memorypolymers doped with long fibers and boards (thin films) must beexplained by bending theory because of the property of large de-formations (see Fig. 4.2).

Fig. 4.2. A typical post-buckling phenomenon: single carbon fiber in the shape memory polymer buckling, showing a similar sine wave form [214].

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Bending refers to the phenomenon that when the structureloses stability, that is, when loading reaches some threshold, thestructure will suddenly switch to another neutral balance. This is aproblem of structure stability that was discovered long ago and hasbeen researched for centuries [215]. Theoretical studies in this fieldare highly mature, dating back to the 18th century, when Euler et al.researched the stability of rods under compression. However,bending analysis in geometry and mechanics, specifically post-bending theory, has been a widespread focus in recent decadesand has become the common focus of subjects including appliedmath, biology, material science, mechanics, physics and engineer-ing [216]. Early studies on bending mainly sought to avoid bendingin the engineering of architectural structures. On the other hand, inrecent decades, scientists have found numerous fantastic cases ofbending in nature. The extensive geometric deformation can ach-ieve new functions [217e219], such as the bending of thin beamsand shells in stretchable electronics [220,221], complex 3-dimensional structures [222,223], materials with negative Poissonratio [224e226] and adjustable acoustic super materials [227,228].Furthermore, the changes in surface curvature produced by wrin-kling and folding, facial chemistry [229], wetting [230], force ofcohesion [231] and resistance can be controlled. With the increasein research pursuing smart structures, bending is a common focusbecause of its unique energetic and geometric properties, which areuseful in applications related to energy (such as sensors, energycollection, damping accessories and drivers) and in the field ofstructures that undergo large deformations.

There are multiple cases in which elasticity loses stability[232e240]. In addition to compression, stretching can also causeelasticity to lose stability to establish a backward-bending shape. Atpresent, smart structures consisting of shape memory polymersmainly use the compression bending of fibers or boards. This essaymainly discusses the conditions of bending the and geometricproperties of backward-bending, which have broad applications inthe design of driving devices and in establishing structures usingshape memory polymers.

A thin membrane with surfaces consisting of large-deformationmaterial will be compressed as the matrix is compressed, and theelasticity will lose stability when the reaction reaches a certainextent, whichmanifests as corrugation of the surface. However, thisstructure strengthens shape memory polymers and does not sac-rifice their applications in structures that undergo large de-formations. On the other hand, nanoscale surface corrugationstructures will introduce new properties to the material, such asphoton structures, anti-drag in fluids, and super-hydrophobicity[241]. Traditionally, this surface structure was achieved by top-down methods, such as photoetching and impression. Thesemethods are relatively comprehensive, but they require complexmachines and are expensive. However, using bottom-up methodscould produce this surface structure efficiently and precisely [54][242e246]. The corrugation and thickness of thin membranes arerelated to dynamic properties such as the shearing modulus, andthus this method is highly designable. Huang et al. reported thebending of semi-conductor membranes on periodically attached

silicon boards [205,247e251]. In addition, when the boards werewarped during compression, nanowires and tubes were fixed topre-loaded boards. In the work of Huang and Zhang et al., thismethod has undergone numerous theoretical studies [252e255],which had significant and broad applications in many fields ofscience, including biomedical devices, MEMS systems and memorystorage systems.

When a thin membrane is compressed on a single axis, it per-forms like a sinusoidal wave in the backward-bending phase. Theperformance on two axes is more complicated and can take theform of a chessboard, a hexagon, a “Y” and various other shapes. Inaddition, the shear force also influences the shape. Shape memorypolymers have special advantages in establishing this kind of sur-face shape, such as unloaded long-term storage and controllableshape recovery. It is obvious that shape memory polymers haveparticularly high potential for application among all kinds of smartstructures that use active deforming materials.

There have been many studies on the bending dynamics oftough surfaces on thin membranes of soft substrates and onbackward-bending shapes [201,256e259]. In 2012, Chen et al.examined the corrugation of coating films of shape memory poly-mers and used viscoelastic models and the principle of lowest en-ergy to analyze the backward-bending shape of surfaces ofcomposite polymers [260]. After experiments examining the in-fluences of the programming and the membrane thickness on thewavelength and amplitude of corrugation, the theory gave goodpredictions within a wavelength range of 930 nm to 5 mm. Lenget al. analyzed the influence of the curvature and thickness of themembrane on backward-bending shapes for shape memory poly-mers with their surfaces strengthened bymetal films and subjectedto large deformations, and they analyzed the surface bending bythe finite-variable method [261]. Furthermore, this group analyzedthe dynamics of one-way fiber-strengthened shape memory poly-mer composite materials after local bending [262]. As shown in thefigure, the horizontal sections under curved bending can be clas-sified into non-bending stretching, non-bending compression andbending compression sections, which are characterized by thecritical position of bending, the position of the neutral surface andthe half-wavelength of optical fibers. Additionally, the critical cur-vature of bending, critical position of bending, position of theneutral surface, wavelength of the fiber bending, amplitude of thefiber bending and macroscopic reactions of the structure werecalculated.

It should be noted that bending could occur only in thin films.For thicker laminates or a larger modulus, the overwhelminglylarge shear force prevents the bending of the fibers or boards. Thematerials appear to bend as a whole, which departs from theresearch focus on bending. However, these cases are still of interest,since they can be used for origami structures and have bright fu-tures in rapid production and self-loading. The dynamic theoremsare relatively simple and will not be introduced here, whereas thedetails of application will be introduced in Section 5.3.

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5. Applications of SMPCs

Applications have always been the focus of studies on shapememory polymers and their composites. During the 1960s, shapememory polymers gained their first large-scale applications,namely, the use of PE thermal contraction tubes [263,264], whichnow are made from nylon [265] or polystyrene [266] materials.Thermal contraction tubes have good flame retardance, insulationand temperature tolerance properties and have beenwidely used ininsulated joint protection, harnesses and erosion prevention, aswell as having broad applications in electrical work. This was themost meaningful large-scale application of shape memory poly-mers and thus attracted scientific research attention. Since then,studies of shape memory polymers have undergone rapiddevelopment.

To develop applications in which shape memory polymers arecentral, we need to know the properties of shape memory poly-mers. First, we consider two-phase shapememory polymers, whichhave abundant useful properties and have been researched mostthoroughly. The essential information is that they exhibit single-direction recovery, that is, in an unloaded recovery process, un-less we reprogram them, shape memory polymers can transformonly from temporary shapes to their original shapes. Even thoughheating or other stimulations during the recovery process arehighly designable, their one-way dynamic behavior still greatlylimits their applications. As a result, we consider using shapememory polymers and their composites only in structures withmainly one-way deformations. One major type of such applicationsaims at solving transportation problems by enabling transportationin a convenient shape and recovery to shapes needed for functionsat specified times, as in the case of spatially expanding structureand minimally invasive surgical instruments. Another significantapplication of shape memory polymers is self-arrangement, inwhich the operation conditions perform cold programming. In thiscase, shape recovery can be utilized to let instruments and struc-tures recover from plastic deformations accumulated during use,thus extending their lifespans.

Additionally, shape memory polymers are a type of elastic-adhesive elastic material, that is, they show normal elasticitywithout stimulation and show adhesive elasticity under stimula-tion, which must be taken into consideration in engineering ap-plications. As shape memory polymers are macromolecularmaterials, their programming and recovery are both creepingprocesses, which makes their shape memory behaviors time-consuming. On the one hand, the reaction time gives smart struc-tures based on shape memory polymers the property of low im-pulse, but on the other hand, it limits applications involving quickreactions.

Shape memory polymers are limited to their lower thermody-namic properties and single stimulation mode, so making shapememory polymer composites are extremely valuable. In the aero-space field, shape memory polymers have a tendency to replaceshape memory alloys and incorporate structural materials withtheir light weight, high deformability and high fixation rate.Through composite fiber materials, the hardness, hardness ratio,recovery forces and other thermodynamic properties could begreatly enhanced while maintaining the above advantages. In thefield of biomedical instruments, shape memory polymers aremainly used in appliances, brackets and occluders. Shape memorypolymer smart structures applied inside the body, such as mini-mally invasive surgical instruments, require methods of stimula-tionwithout direct contact, which are often realized by the indirectheating of doping materials. Furthermore, shapememory polymers

could be used to make substrates, sensors, energy gathering de-vices and soft electronic accessories. Shape memory polymercomposite materials also have special value for applications in thefields of smart fabrics, 4D printing, etc.

5.1. Applications in aerospace and aviation

Because of their stable single-deformation ability, shape mem-ory materials have attracted broad attention in all kinds of aero-space expansion structures and in driving machine-based locking-release structures with limited deformation times. Early studies ofvariant structures, mainly using shape memory alloys to providedriving forces, included a series of applications of spatiallyexpanding hinges [267,268] and variant structures in aircrafts[269e271]. Their relatively large density (6e8 g/cm3) made itdifficult to use shape memory alloys as structural materials, andthey were necessarily accompanied by other light materials in theform of wires or boards, which increased the difficulty of designand introduced the inconvenience of choosing the accompanyingstructural materials. On the other hand, the shape memory effectsof the shape memory alloys themselves were not satisfactory.Therefore, shape memory polymers became a rising star in solvingproblems.

By comparing the properties of fiber-strengthened shapememory polymers to those of shape memory alloys, we directlyobtained ideas of the necessary differences in application design.Because of the low mass, high toughness and large deformation ofpolymer-based shapememory composites, they could bemade intomatrices of large expansion structures (such as reflective antennas[84]), which achieved the combination of driving devices andstructural materials.

In our previous review, the use of shape memory polymers inthe aerospace and aerospace fields has been exhaustively analyzed.In this article, we further clarify the design principles, and intro-duced some typical applications, especially large-scale expansion ofthe structure.

The environment in space is much worse than that in the at-mosphere. Polymers and other organic materials will be moreseverely eroded than metal or ceramic materials by atomic oxygenand ultraviolet radiation and will be subjected to extreme tem-perature differences, which will lead to loss of mass of polymers,reduction in their dynamic properties, or even complete loss offunction [272e276]. As a result, shape memory polymers must becarefully selected and tested for applications in space. For shapememory polymers, these tests should not only consider conven-tional factors such as losses in mass and changes in componentsand modulus but also the loss of the shape memory ability. It hasnow been proven that epoxy resin-based and cyanate ester-basedshape memory polymers have good tolerance of the outer-spaceenvironment. Furthermore, because of their outstanding thermo-dynamic properties (especially the tolerance of high and lowtemperature cycling) and stable chemical properties, polyimide-based shape memory polymers have been identified as ideal can-didates for future applications of shape memory polymers in space.Similarly, fillers with good space tolerance such as carbon materialand glass fiber must be chosen when making shape memorypolymer composites. When doping with materials with activechemical properties, such as healing agents, special design con-siderations are necessary to prevent these active properties frombeing influenced by space radiation (see Fig. 5.1).

Fig. 5.1. (a) Modular shape memory polymer composite space deployment truss. A variety of trusses can be easily produced by this unit [83]. (b,c) Large deployable shape memorypolymer composite structure. (b) Inflatable antennas based on SMPC-based reflector substrate [277].(c) Expandable space accommodation [278].(d,e) Application of shape memorypolymers and composites in morphing aircraft. (d) Morphing aircraft with foldable wings. The skin of the deformable part is made of SMP [279,280]. (e) Steel wire springs areincorporated into the SMP to provide the dual functions of strengthening and heating [281].

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5.2. Biomedical devices based on SMPCs

Shape memory polymers have been of interest to the medicalfield from the beginning, which can be dated back to the fourthdecade of the last century when Vernon et al. first introducedshape memory polymers in a dentistry patent. Compared withmetal materials, polymers offer suitable recovery force as well asgood biological adaptability. Many shape memory polymers alsohave the ability to be decomposed in the body, such as poly-urethane [282], polycaprolactone [17] polyether urethane [12]and polylactic acid [283,284]. On the other hand, by makingcomposite materials, the driving methods of shape memorypolymers can be enriched. Among them, light driving (especiallymicrowave) and magnetic driving offer the ability of remotedriving, whose principles and examples have already beenintroduced in Section 3.2 and will not be restated. These twoproperties give shape memory polymer composites excellentpotential for use in medical instruments used inside the body.Shape memory polymers and their composite materials havebeen used in many biomedical instruments, as described in thereports of prototypes and experiments. Their uses includethrombus cleaners, surgical sutures, intravascular stents, aneu-rysm occluders and orthodontic fixers.

Buckley et al. made original flower-shaped and inflated ball-shaped operation instruments using SMP and SMP foam withmagnetic doping, which could be controllably expanded in thebody through magnetic driving [285] (Fig. 5.2(a)). Small et al.wrapped a layer of shape memory polymer around fiber to achieveIR-stimulated shape recovery [286]. These indirect drivingmethodslaid the foundations for the use of shape memory polymers forin vivo surgical instruments. Ali et al. used a double-sided Cu-coated polyimide to produce a magnetically stimulated shapememory polymer drug release mechanism based on simple heatingstimulation [287].

Lendlein et al. reported a surgical suture made of shapememory polymers, whose temperature of transformation wasnear body temperature [17]. Experiments on animals illustratedthat the suture underwent shape recovery when the temperaturerose to 41 �C, closing the surgical incisions. Wache et al. devel-oped a new concept of stents using shape memory polymers [20].This stent could be inflated to eliminate the problem that tradi-tional metal stents limited the transportation of medicine bymaking vessels narrower. Shandas developed a porous stent,which further minimized the necessary size of the trans-formation and provided convenience for minimally invasivecardiovascular operations [288]. Lendlein et al. reported a ureterstent made of decomposable shape memory polymers, whichcould provide directional dosing and decomposition [21]. Thisgroup also designed a directional dosing structure made of shapememory polymers, whose shape recovery could be realized bysingle-step or multi-step methods and which achieved the pre-dictable release of an effective loading of 90% or more in 80 days[289,290]. Cho et al. produced shape memory polymer floss fororthodontics using polyurethane base shape memory polymer bymelt spinning. The balance recovery force was approximately 50gf, which was sufficient to correct unadjusted teeth in ortho-dontic tests [291]. Yakacki et al. reported a decomposable shapememory polymer with a modulus as high as 23.0MPa and used itto make soft tissue repair materials [292].

Hampikan et al. made aneurysm coils from shape memorypolymers to replace traditional platinum coils, eliminatinganeurysm re-opening caused by biological inertia [293](Fig. 5.2(b)). The tantalum filler is compounded in a shapememory polymer, increasing the X-ray impermeability, therebyobtaining an X-ray image of the tantalum filled SMP coil.Through the simulation experiment, it is proved that theresponse of the coil does not affect the blood flow. Maitland et al.designed a medical stent to increase the visibility of X-ray ormagnetic resonance (MR) imaging by doping with iron nano-particles or Gd chelates [294]. Metcalfe et al. reported apolyurethane-based foam occluder, which was proven by animalexperiments to occlude aneurysms effectively [264]. The major-ity of aneurysm necks were sealed by the formation of a thicknew internal membrane on the surface of foam bubbles. Smallet al. designed a new embolization structure made of stents andfoam [295]. Both parts consisted of shape memory polymers, andthe structure was used to address and cure more difficult wide-necked aneurysms. Maitland made a new kind of aneurysmembolization structure using an inflatable SMP foam [296,297].Platinum coils or a deployable alloy stent was used foranchorage, and the SMP foam could be inflated to over onehundred times its original volume (Fig. 5.2(c)). Hernandez et al.have designed a new hemostatic device made of shape memoryfoam that can rapidly fill wounds to achieve hemostasis andsterilization. The device expansion force is very small and willnot cause secondary damage to the wound [298] (Fig. 5.2(d)).

5.3. Two types of enlightening applications: smart textiles,microelectronics

In addition to the particular focus on the aerospace andbiomedical fields, shape memory polymers could also be appliedto fields such as material shaping, smart fabrics, sensors, energycollecting devices, and soft electronic device substrates, amongothers.

For complicated structures, especially for shaping bent com-posite material structures, the making of dabbers is a challenge.Traditionally, there are two solutions: multi-piece metal man-drels and water-soluble mandrels. The former can ensure goodprecision and repeatability, but the design, production, installa-tion and uninstallation are expensive. The latter are seriouslylimited because of disposability and hazardous residues. At thistime, shape memory polymers have become a candidate forsolving the problem. Compared with alloys, shape memorypolymers and their composite materials have greater deforma-tion ability, which makes it easier to produce more complexstructures. The CRG company used shape memory polymers toproduce bottle-shaped and S-shaped air duct structures, whosedabbers could be used repeatedly and were easy to knock out,and offered ideas for making complex bent structures [299e301].Leng et al. further designed shape memory polymer dabbersusing the dynamics of composite materials [302,303]. Finitevariable analysis and illustrative experiments showed that shapememory polymer dabbers had good deformation and repeat-ability. However, the transformation temperature of shapememory polymers, which has been researched somewhat

Fig. 5.2. Biomedical devices based on SMPCs. (a) Flower-shaped surgical instruments made of SMPC with magnetic doping. (b) Aneurysm coils based on shape memory polymerswithout platinum coils. (c) Aneurysm embolization structure based on inflatable SMP foam, which can expand more than a hundred times. (d) Hemostatic device based on shapememory foam that can rapidly fill a wound to achieve hemostasis and sterilization [267,275,278,280].

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Fig. 5.3. Items made of shape memory polymers: pillows and insoles [22].

Fig. 5.4. Shape memory polymer-based self-healing triboelectric nanogenerator. (a)Schematic of the PUeTENG structure. (b) PU pyramid pattern on the PU triboelectriclayer [24].

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198188

thoroughly, still presents difficulties when applied to materialsthat require higher temperatures for shaping. The choice ofmaterials is a further direction for research on shape memorypolymer dabbers.

Shape memory materials have the property of self-finishing,that is, the fatigue and injury introduced in the use of shapememory material devices or by various complex physical orchemical factors in the working environment, especially thematerial deformation caused by external forces, can be regarded

to some extent as programming processes of the shape memorymaterials. This means that the materials could be recovered totheir original shapes under stimulations such as heating, or asthe saying goes, “the phoenix reaches nirvana and rises from theashes”. This property has been applied to the smart fabrics fieldand to self-healing devices. Shape memory polymer fibers can beobtained by moist spinning [304], melt spinning [305] or electricspinning [306e308] to make textiles or non-woven fabrics. Themain reason for wrinkling in textiles is the lack of adjustment ofhydrogen bonds. By making textiles from shape memory poly-mers, wrinkles could be automatically removed through stimu-lations such as heating [22,309]. Furthermore, self-finishinginsoles and pillows could be made using shape memory polymerfoam [22,291], Some examples are shown in Fig. 5.3.

Kim et al. reported a new kind of triboelectric nanogenerator(TENG), with a frictional electric layer made of shape memorypolymers [24]. A diagram and a photo of the structure are shownin Fig. 5.4. After its pyramid structure is worn down by long-termuse, shape memory effects could allow the devices to regain theiroriginal properties, extending the lifespans of the TENGs. Thisapproach is an effective way to extend the lifespans of electronicor mechanic devices.

In addition, shape memory polymers are used in soft elec-tronics. Soft electronics is a field focused on biological electronictechniques. Traditional hard electronic devices cannot adjust tocomplex and soft biological surfaces, and soft electronic devicesexhibit better properties and cause lower discomfort in use[291e311]. Compared to normal soft electronic devices, there is abenefit to using shape memory polymers. That is, their hardnessis changeable. When heated, the shape memory polymer is in asoft and programmable state, which could allow the adjustmentof its properties or its self-finishing. When the temperature is

Fig. 5.5. 3D printing product of shape memory polymers and composites. (a) 3D printing of the Eiffel Tower based on shape memory polymers, with meticulous detail andoutstanding ability to recover. (b) SMPC 3D printing based on ESBO and BFDGE, with carbon nanofibers (CNFs) added as fillers to tailor the mechanical properties and Tg whileinducing electrical conductivity. The scale bar is 1 cm. (c) 3D printing of SMPC doped with ferrite to achieve remote control of the shape change. (d) Twenty truncated printingsurface created by stereolithography apparatus (SLA) showed good shape memory properties under both cutting in flattening and flattening cases [302e323].

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reduced to room temperature, it enters a tough and stable state,which can stabilize its properties and offer structural supports.Voit et al. made 3-dimensional stimulation electrodes usingshape memory polymer substrates whose hardness could changeby over 100 times for automatic adaptation to the geometricshapes in the bodies of creatures [312]. Animal experimentsproved that this electrode effectively avoided inconsistency be-tween biological and non-biological surfaces [294]. This groupfurther made crystal tubes with organic thin membranes usingshape memory polymers through full photolithography [313].This system exhibited excellent electric properties and reliabilityand retained its function after 100 cycles of softening andbending. Leng et al. incorporated gold electrodes into shapememory polymers. Experiments showed that the resistance wasan exponential function of the reactance. These structures couldbe used to make adjustable resistors or sensors [314]. In addition,Luo et al. reported that resistors containing carbon nanotubesand shape memory polyurethane composite material showedsensitivity to water and thus could be used to make moisturesensors [315]. Furthermore, this group combined surface layers ofshape memory polyurethane with silver nanoscale wire (AgNW)and observed that the resistance of the composite materials wassensitive to temperature, enabling the construction of soft tem-perature sensors [316]. Zhong et al. used layer-by-layer (LBL)techniques to make a new type of super capacitor from multi-walled carbon nanotubes and shape memory polymers, whichprovided excellent stretchability, shape memory capacity andenergy storage properties [317]. Kipplen et al., using wearablebiologically consistent shape memory polymers as substrates,developed a green electroluminescent phosphorous organicdiode, which had excellent lighting and self-finishing ability[318].

5.4. A potential share: 4D printing and origami

Shape memory polymers also have some status in self-installation fields. Macroscopic independent installation is aconcept with strong prospects. This creative thought not only ishighly interesting but also would reduce storage space andtransportation, and its low cost and fast production give it greatpotential for development. The changes in shape of most shapememory polymers are permanent and stable. Even though theylose the ability to perform repeated driving, this property alsomakes them irreplaceable structural materials that achievepermanent shapes in independent installation. Using conve-nient, quick and large-scale handling methods, simple structurescould be processed into complicated shapes. One idea is 4Dprinting.

4D printing is an improvement of 3D printing, introducingtime into printed matter. And the change involves not only shape,but other properties tuning. They are all design directions (e.g.optics property). This change is also active, so 4D printing isactually the printing of smart materials containing time-dependent.4D printing has gradually become a hot point, just

recently, International Journal of Precision Engineering andManufacturing-Green Technology [319] 4D published a mono-graph of 4D printing, which includes more than 6 articles about4D printing review, so this paper will not repeatedly introduce indetail.

4D prints have unique or even irreplaceable advantages inmany fields. For example, through the rational design shapes ofidle and application, efficiency of production and transportationcan be improved. Another example is the production of soft drivecomponents, 4D print product can be used directly without theneed to introduce complex mechanical components[27,320,321].

Magdassi et al. pioneered 3D printing of shape memory poly-mers [27] (Fig. 5.5(a)), and Lewicki et al. achieved 3D printing ofshape memory composites [303] (Fig. 5.5(b)). Choong et al. usedstereolithography apparatus (SLA) and modified two-componentshape memory polymer to print a truncated twenty face bodyframe. Cut it off and flatten it, they get a pretty recovery result [194],whichmakes the practitioner feel confident (Fig. 5.5 (d)). Leng et al.achieved the 3D printing of polymers with excellent shapememoryability by the direct-write printing of UV crosslinking poly(lacticacid)-based inks dopedwith ferrite to achieve remote control of theshape change [322] (Fig. 5.5(c)).

Paper art is a kind of paper processing, or more abstractly, isthe processing of the plane, so that the plane will get structurebeyond the plane: Origami makes the structure develop fromtwo dimensions to three dimensions, and kirigami increases thedegree of freedom of plane. So paper art can be said a 3D printingprocess.

Traditional origami needs to be folded manually, allowing thematerial to yield at creases. It's a passive origami, it must be donestep by step. And more importantly, it can't be printed. When aprogrammable material or composite system is used, origami be-comes active, and becomes a printing deformationmaterial. (Activecompletion does not mean that it should be done by one step, andactive folding step by step can create complex structures such asriveting [26].) When printing deformable materials are introduced,origami has shown new vitality in the field of rapid flexiblemanufacturing. We can say that origami is such a simple andpractical 4D printing method.

For 4D printing, the concept of active origami for flats hasbeen reported, which is the idea of producing 3-dimensionalstructures from a 2-dimensional sheet. Quite complicatedfunctional structures and microscopic structures have beenmade. Qi et al. incorporated shape memory polymer fibers intoelastic polymer substrates, whose curvature was controlled byadjusting the reactance of the stretching [25,324e329]. Now thestructure can realize complex structure, such as Miura origamiand delicate origami [330]. Using this basic idea, a series of 4Dprinting structures were designed. Inspired by pop-up structuresin MEMS [331e333], Felton et al. made multi-layer compositeboards using pre-stretched shape memory polymers and createda series of origami structures [26,119,334] whose sizes reachedas low as microscale [120]. We are looking forward to thedevelopment and maturity of self-installation (see Fig. 5.6).

Fig. 5.6. Origami is achieved by laminating an SMPC. (a) Schematics of a PAC hinge and programming progress. In essence, this folding is also a loaded shape memory cycleproduced by stretching at high temperatures. After the external load is released, the hinge is bent to the designed angle because of the competition between the base material andthe shape memory material. Afterward, the hinge can be reheated and reused. (b) Creation of a movable robot based on origami. The figure shows the robot at each stage ofassembly, three of which are self-folding. (c) A boat is made by a similar method, and its size is as low as the millimeter level [26,120,309].

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 191

Fig. 5.7. (a) Several shapes inspired by the native orchid on the right are printed by hydrogel composite with cellulose and swollen in water. The scale is 5mm(b) Tulip, printed withshape memory polymer. From left to right are permanent shape, temporary shape and shape of recovery. Which light blue and dark blue corresponding to 2 s and 4 s of exposure,respectively. The scale is 1 cm [317,318]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198192

Lewis et al. inspired by the plant, designed a printing methodthrough cellulose-based hydrogel composite [335]. In different lo-cations, the proportion of cellulose in the axial and normal orien-tation is different, so water swelling capacity is different. In thisway, the flat print can be designed to swell into the correspondingshape (Fig. 5.7 (a)). Unlike above origami, the three-dimensionalbuilding obtained in this case is a non-developable surface. Zhaoand Xie et al. also used the global deformation to design a 4D printbased on swell equilibria [336], which utilizes ultraviolet light fordifferent exposure time, allowing the hydrogels to achieve differentcross-linking conditions, thereby changing the swell equilibria. Thetechnology exposes the plane simultaneously, no longer rely onone-dimensional print head. Further, the group replaced thehydrogel with shapememory polymer (hydrophobic lauryl acrylate(LA) was used as themonomer,1,6-hexanediol diacrylate (HDDA) asthe cross-linker), and swelled in a wax (Fig. 5.7 b)). Benefited fromthe shape memory effect, which is obtained by the shape that canbe fixed while the hydrogel cannot. Shape memory performance isexcellent, the fixation rate of up to 100% and recovery rate of up to95%.

The above methods can be designed by program calculationand manufactured by programmatic printing. Their production isconvenient and fast. Therefore, we have reason to believe thatthis approach will lead to a revolution in production techniques.

6. Conclusion and outlook

It is not appropriate to discuss shape memory polymers withoutdiscussing shape memory effect, so I hope that increasing numbersof people will be able to learn about the shape memory effect as ageneral phenomenon of polymer mechanics that occurs throughthe material learning of shape memory polymers, i.e., it is deter-mined by the properties of the material and the external factors

together. This concept can guide their design, especially in cases ofactive deformation behavior. We distinguish and classify shapememory polymers and shape memory behavior mainly because ofdifferent focuses in materials science and engineering.

Shape memory polymers and their composites are excellentsmart materials with outstanding properties and rich functionality.The development of composite materials (SMPCs) has greatlyexpanded the functionality and application range of shape memorypolymers. In this paper, the flourishing of SMPC materials has beenreviewed from the perspective of material function and application.Doping has been found to overcome the defects of shape memorypolymers, including the minor restoring force and the limitedstimulation methods, which enables the application of SMPCs inmany fields from aerospace to biomedicine.

While appreciating these developments, we must also addressmany daunting challenges:

1. Althoughwe have seen awealth of development ideas involvingshape memory behavior, the shape memory polymer is essen-tially limited by the great shortcoming of its own one-waydeformation. The shape memory effect must be extended totwo-way or combined with other active deformation principles,or shape memory polymers will be trapped in a small applica-tion area, such as deployable structures. We have seen thatcomposites have made some initial progress in solving thisproblem (See Section 3.3 "Creating SMPCs with novel shapememory behavior"), and it is important to examine how com-posite concepts can achieve greater freedom or even full two-way recovery.

2. The electric driving of SMPCs is the stimulationmethodwith thegreatest potential in application, and its research is also themostabundant. Despite substantial encouraging progress, we mustadmit that electric driving is still in the experimental stage. Its

T. Mu et al. / Composites Science and Technology 160 (2018) 169e198 193

reliability is not high enough, and the cost is relatively high.Currently, we still use paste or buried resistance film for elec-trical heating, as we lack reports on the electrically stimulateddevices and structures using the shape memory polymernanocomposites. How to produce electrically stimulated SMPCswith high reliability and low cost is still the challenge we face.

3. We see in Section 3.1, "Reinforced SMPCs", that in most cases,when SMPs are changed to SMPCs, we can obtain better me-chanical properties, but the shape memory effect will bereduced, mainly reflected in the fixation rate and the recoveryrate. Especially for long-fiber-reinforced shape memory poly-mer composites, the most promising SMPC in engineering, astrong fiber will lead to a substantial decline in the fixation rateand recovery rate, a decrease in the transition temperature, andthe development of non-negligible creep during storage. Thissituation is often difficult to solve with limited strain. However,sufficient stiffness is vital to the application of shape memorypolymers in the field of drives. Therefore, a solution to thisproblem is essential for engineering applications of SMPCs.

In addition to these long and arduous challenges, we have seensome opportunities whose research is still in its infancy, such as thefollowing:

1. The shape memory polymer itself and its auxiliary self-healinghave great attraction in abundant multifunctional shape mem-ory materials, but it is still a young research direction. Althoughthere have been many studies of principle, today, due to thelimitations of mechanical properties, cost, and healing effects,the progress of self-healing is still in the experimental stage, sothe self-healing route in use in practical engineering applica-tions remains long. In addition, other multifunctional materialsand composites, including combinations of the shape memoryeffect with soft electronics, material health monitoring, energycollection and management, and radar-absorbing materials,have unique applications in electronic technology, aerospace,defense and so on. They offer promising future directions.

2. The use of shape memory polymers to create microstructuresremains an area to be developed, particularly for the creation ofmicrostructures inside shape memory polymers, as well as theincorporation of a polymer basis or polymer doping into newfunctional materials. Shape memory polymers have the abilityto deform from the nanoscale to the macroscopic scale, somicro-materials designed with shape memory polymers willhave tunable properties. In addition, shape memory polymercomposites are unique in creating microstructures, such as bythe use of composite filler materials as sacrificial components,and by the creation of microstructures by buckling. 3D printingtechniques for shape memory polymers and their particle-doping composites have been studied. This is an importantmethod for creating complex 3D structures, and the multi-component printing of SMPC can be used to produce novelfunctional materials.

3. For engineering applications, not only do shape memory poly-mers and their composites provide an alternative to shapememory alloys, which have been extensively designed and usedin the past, but also their large deformability allows SMPs orSMPCs to fabricate structural materials from large deformablestructures that were unimaginable in the past, such as parabolicreflection antenna. We should note that the greatest advantageof shape memory polymers is not the replacement of thebending mechanism in traditional structure but their benefits incurve and surface structures. We should break the constraints oftraditional design ideas to design more ingenious and flexiblestructures and applications.

In recent years, shapememory polymers and the shapememoryeffect have become increasingly familiar to materials scientists andhave attracted their research interest. SMP and SMPC research hasundergone rapid and even explosive growth. We have reason tobelieve it will soon be even more vigorous.

Acknowledgement

This work is supported by the National Natural Science Foun-dation of China: Grant Nos. 11672086, 11772109 and 11632005.

Thanks to Xisu Wang for translating and collating some of themanuscripts in English. Thanks to Hongrui Suit finishing the articleand the reference document format.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.compscitech.2018.03.018.

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